BCO and Cooker Circuit Update

[TheScullster]

Hi all

OK so now I'm struggling...........

Rang the BC inspector this morning to point out the allowed 15Kw rating for cooker circuit of 32A. He says yes that's OK but you've got to do the calculations for the cable sizing to Appendix 4. His interpretation is that the cable size has to be assessed on the maximum current rating of the attached device(s), not on the basis of the de-rated first 10A + 30% remainder. It seems pretty clear to me that this is not the case, based on the 15Kw/32A circuit example given in the various OSGs etc. His argument was based around the fact that a 6Kw oven could be supplied with a 1.5mm cable clipped direct (based on the first 10A rule) - but "you wouldn't do it".

So where do I go from here? Can I contact the IEE directly to get a ruling? Why do all my jobs seem to hit obstacles like this?

I am reluctant to take up Adam's offer of "supplying the vocals", as this guy seems fairly firmly entrenched.

Thanks

Phil

[Dave Plowman (News)]

Is he the only BC inspector in your area? I'd be inclined to ask for a different one if not. The guy clearly has a bee in his bonnet.

[John Rumm]

Well you could follow the approach given in:

how to size a cable when overload protection is required.

This states that the cable will need to be able to handle a sustained load of 1.45 x the MCB's nominal breaking current. (as the MCB has a lower rating than the appliance, it is the MCB which will impose the limit on the maximum demand in this case).

Hence in this case the cable should be able to sustain 1.45 x 32 =

46.4A, which is within the tabulated capability of 6mm^2 clipped direct.

Column C from:

His argument was based around the fact that a 6Kw oven could be supplied

Well given that in normal circumstances a cooker circuit would often be wired in 4.0mm^2 T&E, it does not seem too onerous to go to 6mm^2 to keep him happy. (in fact 6.00mm^2 on cooker circuits is not that uncommon IME)

[Andy Wade]

Yes, apply Appendix 4 using the design current (Ib) *after* any applicable diversity. Include all relevant factors for ambient temperature, grouping, thermal insulation, rewireable fuse, etc. Also check that voltage drop and Zs are within the prescribed limits (The OSG table of max. circuit lengths takes care of that in most cases.)

That's just plain wrong.

You wouldn't, but that's not a good example. Generally the FLC is dominated by the hob, not the oven(s) and the probability of all rings running flat-out for a significant length of time is negligible.

When using the standard diversity rules you should always consider whether they are reasonable in the circumstances. In your case (hob plus single oven) they manifestly are. If you had a giant oven and a two-ring hob it would be different matter.

You already have that in the OSG and EGBR. Assuming you've provided 6 mm^2 cable and a 32 A MCB I can't see why there should be a problem. You could take up the issue with the guy's competent person scheme, assuming he's a member of one. They all have technical support lines.

[chris French]

In message , TheScullster writes

Can you speak to his boss?

[ARWadsworth]

I was wondering if that article could actually be altered we only need "Iz >= In "

[ARWadsworth]

Well with a 20A MCB you could. That is the whole point. And it would work and be safe as both ovens will not be on for long.

I doubt that they would be interested.

Can you just stick a 40A MCB in to keep the jobsworth happy?

Maybe a few of us could write a letter to him?

[ARWadsworth]

I suspect that the trouble is that this BCO is actually used for all electrical inspections in the area and knows more about electrics than his boss. His boss would need to see the scans that I posted last night and be given an explanaton of why they are relevant before he can overule his BCO.

[TheScullster]

"ARWadsworth" wrote

Yes this guy covers the whole area unfortunately.

Phil

[ARWadsworth]

I wonder if Hugo Nebular knows how you can appeal against the BCOs decisions? You might have to ping him.

[John Rumm]

Does one need to? If you go through the calculations as he requested, but use the non diverse loading, you still come out with a solution using 6mm^2 in all probability. That does not seem to be too difficult to implement on the end of a 32A MCB.

[TheScullster]

"ARWadsworth" wrote

Unfortunately 40A mcb still doesn't cut it when looking at the published loads for these devices. Looking at Bosch appliances, double oven 4400-5500w, hob 6600-7900w. Therefore even the lowest spec double oven and hob combo cannot be accommodated using even a 45A breaker! Is a 45A Hager mcb still available? If so this route might just cover a single oven.

[TheScullster]

"Dave Plowman (News)" wrote

Unfortunately it appears he covers this whole area!

Phil

[TheScullster]

"John Rumm" wrote

Hi John

The cable size *is* 6mm - it's this guy's refusal to use the current draw after "diversity" (or more accurately the "assumed current calculation") has been applied that is the issue here.

Phil

[TheScullster]

"Andy Wade" wrote

OK Andy, this is the crux of the matter - where do the regs state that appendix 4 should be applied *after* the reduction for diversity (or "assumed demand" calculation) has been applied?

[TheScullster]

"John Rumm" wrote

Unfortunately not John - see my reply to Adam's post.

Phil

[TheScullster]

"TheScullster" wrote

Sorry last line should read single oven and hob.

Phil

[ARWadsworth]

Hide the cooker and the oven in the bathroom when the BCO comes back and let him test to the outlet plates:-)

[TheScullster]

"ARWadsworth" wrote

Nice idea, but his interest in this area is aroused and he says he wants to inspect the completed kitchen in any case. Also, the fact that the carcase would have a double-oven-size hole might be a give away.

Phil

[Andy Wade]

In the 16th ed. (since you're using that) it's 311-01-01. "... In determining the maximum demand of an installation *or part thereof,* diversity may be taken into account. [my emphasis]